Polymer insulators are heretofore known in which housing or envelope of umbrella type as insulating member is attached to a rod formed from resin, onto both ends of said rod are mounted respective metal fixture means. Such polymer insulators are used in various applications such as suspending or retaining use in which only tensile strength is applied as well as in post-insulators or long-stem insulators in which not only tensile strength but also bending moment are applied.
FIG. 1 is a schematic view of fixing portion of a rod onto a fixture means in an example of heretofore known polymer insulators. In said polymer insulator 1, an insulating housing 2 of umbrella type (shown only in its part) is fixed onto an end of a rod 3 formed from resin such as FRP, said end of the rod 3 being mounted into an inwardly tapering hole 5 processed with parting agent and formed on one end of a fixture means 4 made from metal such as black heart malleable castings. Adhesive material 6 such as epoxy resin which can be joined only to said rod 3 is filled between the outer surface of rod 3 and the inner surface of said tapering hole 5, thereby to form a draft-preventing member 7. In practice, soft adhesive material 6 before its setting is injected into said bottom surface 5a of tapering hole 5, and rod 3 is then pushed onto said bottom surface 5a of said hole 5, thereby to push out said adhesive material 6 so as to form a wedge form between the outer surface of said rod 3 and the inner surface of said tapering hole 5. Subsequently, said adhesive material 6 is left to set so as to cause said adhesive material 6 to join only onto said outer surface of said rod 3 thereby to form said draft-preventing member 7. Further, a counter-tensile charge P which is larger than the greatest charge in use is beforehand applied in the direction shown by the arrow in FIG. 1, so as to generate an urging force in the direction separating said rod 3 and said fixture 4 from each other. As a result, a gap 8 is formed on said bottom surface 5a of said tapering hole 5 by causing said rod 3 to shift in said direction of said arrow due to said counter-tensile force P.
In the polymer insulator 1 as described above, when a tensile force is applied on said polymer insulator 1 in a direction such as to separate said rod 3 and said fixture means 4 from each other, said draft-preventing member 7 exhibits its wedge effect in order to prevent said rod 3 from drafting from said fixture means 4.
The polymer insulator 1 having a draft-preventing fixture 7 as shown in FIG. 1 shows no problems when used for suspending or retaining application wherein only the tensile charge is applied thereon. On the contrary, when used as post-insulator or as long-stem insulator wherein bending moment M is repeatedly applied as shown by the dashed line in FIG. 1, said rod 3 shifts in a direction counter to said counter-tensile direction P (as called usually "Rod Shift"), thereby to form a gap (usually called "Play") between the inner surface of said tapering hole 5 and the outer surface of said draft-preventing member 7 formed by said adhesive material 6. Generation of this Play increases oscillation of said rod 3 caused by bending moment M which has edges of opening of said tapering hole 5 and corresponding portions of outer surface of said rod 3 as its supporting points, so that said inside surface of tapering hole 5 and said outer surface of said draft-preventing member 7 not only slide with friction on each other, but also at least outer surface of said draft-preventing member 7 wears from the side of its bottom surface 5a, causing thus its relaxation, with the final result that said rod 3 is drawn out from said fixture means 4.
For the purpose of solving the problems described above, a structure as shown in FIG. 2 is conceivable. FIG. 2 is a schematic view of fixing portion of a rod onto a fixture means. This structure has been designed for increasing the fastening force between rod and fixture means with respect to the embodiment shown in FIG. 1. In said polymer insulator 1A, an insulating housing 2A of umbrella type (shown only in its part) is fixed onto an end of a rod 3A formed from resin such as FRP, said end of the rod 3A being mounted into an inwardly tapering hole 5A formed on one end of a fixture means 4A made from metal such as black heart malleable castings. An end portion of said rod 3A has therein a splitting groove 9 having substantially parallel groove edges for dividing said rod 3A in a diametrical direction. A wedge 10 fixed to substantially central portion of said bottom surface 5Aa of said tapering hole 5A is inserted into said splitting groove 9, and an adhesive member 6A such as epoxy resin is filled in the gap between said splitting groove 9 and said wedge 10, with said adhesive member 6A joining only to said rod 3A. When the end portion of said rod 3A is inserted into said tapering hole 5A of said fixture means 4A, said wedge 10 is also inserted into said splitting groove 9, thereby to enlarge the diameter of said end portion of said rod 3A up to the internal surface of said tapering hole 5A, simultaneously with filling the gap between said splitting groove 9 and said wedge 10 with said adhesive member 6A for preventing said drawing out of said rod 3A from said fixture means 4A. Further, in this polymer insulator 1A, a counter-tensile charge P which is larger than the maximum charge in use is also beforehand applied in the direction shown by the arrow in FIG. 2, so as to generate an urging force in the direction separating said rod 3A and said fixture 4A from each other. As a result, a gap 8A is formed on said bottom surface 5Aa of said tapering hole 5A by causing said rod 3A to shift in said direction of said arrow due to said counter-tensile force P.
In said polymer insulator 1A as shown in FIG. 2 and described above, its anti-wear performance is increased since enlarged end portion of said rod 3A bears directly against the inner surface of said inner surface of said tapering hole 5A. However, this type of polymer insulator 1A has the following inconvenience.
As described above, said adhesive member 6A should be formed from material which can be joined only to said rod 3A, not to said wedge 10, because said adhesive member 6A must separate compulsorily said rod 3A from said wedge 10 when said counter-tensile charge P is applied. This means that it is impossible to join integrally said rod 3A and said fixture means 4A by means of said adhesive member 6A. Consequently, when said counter-tensile charge P is applied, the end portion of said rod 3A which has been previously enlarged by pushing it sufficiently is now reduced again in its diameter, with the result of reduced fastening force between the outer surface of said rod 3A and the inner surface of said tapering hole 5A.
Further, a gap 8Aa is generated between said wedge 10 and said adhesive member 6A which is separated from said wedge 10, and this gap 8Aa allows for inward deformation of two branches of the end portion of said rod 3A, also bringing about reduced fastening force between the outer surface of said rod 3A and the inner surface of said tapering hole 5A.
Further, in practical use, when a charge which is larger than the maximum charge in use estimated for the polymer insulator or than counter-tensile charge P is applied thereto, or when bending moment M has been frequently applied thereon, said gap 8Aa becomes larger, such that said end portion of rod 3A which has been enlarged said wedge 10 will be again reduced, with the result that said rod 3A is finally drawn out from said fixture means 4A.
A further inconvenience is that, since radius of curvature of conical inner surface of said tapering hole 5A is significantly larger than that of enlarged end portion of said rod 3A as shown in FIG. 3, engaging surface area between the outer surface of the enlarged end portion of said rod 3A and the inner surface of said tapering hole 5A is strictly limited, thus bringing about a reduced wedge effect. Further, when bending moment M is generated between said rod 3A and said fixture means 4A, a sliding is caused at said engaging surface area, and/or a large effort is caused between angle portions of said rod 3A on the side of its groove 9 and the slanting surface of said wedge 10 as shown in the circle X of FIG. 3, thus bringing about crack and flaw starting from this portion in said rod 3A.
As is clear from the above description, stable fastening condition between rod 3,3A and fixture means 4,4A can not be maintained for a long period in any of polymer insulators 1 or 1A.